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[InstCombine] Drop nuw flag when CtlzOp is a sub nuw #91776

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May 13, 2024
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[InstCombine] Drop nuw flag when CtlzOp is a sub nuw #91776

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3551109
[InstCombine] Add pre-commit tests for PR91691. NFC.
dtcxzyw May 10, 2024
2f5bad2
[InstCombine] Drop nuw flag when CtlzOp is a sub nuw
dtcxzyw May 10, 2024
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14 changes: 12 additions & 2 deletions llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3342,7 +3342,8 @@ Instruction *InstCombinerImpl::foldSelectOfBools(SelectInst &SI) {
// pattern.
static bool isSafeToRemoveBitCeilSelect(ICmpInst::Predicate Pred, Value *Cond0,
const APInt *Cond1, Value *CtlzOp,
unsigned BitWidth) {
unsigned BitWidth,
bool &ShouldDropNUW) {
// The challenge in recognizing std::bit_ceil(X) is that the operand is used
// for the CTLZ proper and select condition, each possibly with some
// operation like add and sub.
Expand All @@ -3365,6 +3366,8 @@ static bool isSafeToRemoveBitCeilSelect(ICmpInst::Predicate Pred, Value *Cond0,
ConstantRange CR = ConstantRange::makeExactICmpRegion(
CmpInst::getInversePredicate(Pred), *Cond1);

ShouldDropNUW = false;

// Match the operation that's used to compute CtlzOp from CommonAncestor. If
// CtlzOp == CommonAncestor, return true as no operation is needed. If a
// match is found, execute the operation on CR, update CR, and return true.
Expand All @@ -3378,6 +3381,7 @@ static bool isSafeToRemoveBitCeilSelect(ICmpInst::Predicate Pred, Value *Cond0,
return true;
}
if (match(CtlzOp, m_Sub(m_APInt(C), m_Specific(CommonAncestor)))) {
ShouldDropNUW = true;
CR = ConstantRange(*C).sub(CR);
return true;
}
Expand Down Expand Up @@ -3447,14 +3451,20 @@ static Instruction *foldBitCeil(SelectInst &SI, IRBuilderBase &Builder) {
Pred = CmpInst::getInversePredicate(Pred);
}

bool ShouldDropNUW;

if (!match(FalseVal, m_One()) ||
!match(TrueVal,
m_OneUse(m_Shl(m_One(), m_OneUse(m_Sub(m_SpecificInt(BitWidth),
m_Value(Ctlz)))))) ||
!match(Ctlz, m_Intrinsic<Intrinsic::ctlz>(m_Value(CtlzOp), m_Zero())) ||
!isSafeToRemoveBitCeilSelect(Pred, Cond0, Cond1, CtlzOp, BitWidth))
!isSafeToRemoveBitCeilSelect(Pred, Cond0, Cond1, CtlzOp, BitWidth,
ShouldDropNUW))
return nullptr;

if (ShouldDropNUW)
cast<Instruction>(CtlzOp)->setHasNoUnsignedWrap(false);

// Build 1 << (-CTLZ & (BitWidth-1)). The negation likely corresponds to a
// single hardware instruction as opposed to BitWidth - CTLZ, where BitWidth
// is an integer constant. Masking with BitWidth-1 comes free on some
Expand Down
36 changes: 36 additions & 0 deletions llvm/test/Transforms/InstCombine/bit_ceil.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -284,6 +284,42 @@ define <4 x i32> @bit_ceil_v4i32(<4 x i32> %x) {
ret <4 x i32> %sel
}

define i32 @pr91691(i32 %0) {
; CHECK-LABEL: @pr91691(
; CHECK-NEXT: [[TMP2:%.*]] = sub i32 -2, [[TMP0:%.*]]
; CHECK-NEXT: [[TMP3:%.*]] = tail call range(i32 0, 33) i32 @llvm.ctlz.i32(i32 [[TMP2]], i1 false)
; CHECK-NEXT: [[TMP4:%.*]] = sub nsw i32 0, [[TMP3]]
; CHECK-NEXT: [[TMP5:%.*]] = and i32 [[TMP4]], 31
; CHECK-NEXT: [[TMP6:%.*]] = shl nuw i32 1, [[TMP5]]
; CHECK-NEXT: ret i32 [[TMP6]]
;
%2 = sub nuw i32 -2, %0
%3 = tail call i32 @llvm.ctlz.i32(i32 %2, i1 false)
%4 = sub i32 32, %3
%5 = shl i32 1, %4
%6 = icmp ult i32 %0, -2
%7 = select i1 %6, i32 %5, i32 1
ret i32 %7
}

define i32 @pr91691_keep_nsw(i32 %0) {
; CHECK-LABEL: @pr91691_keep_nsw(
; CHECK-NEXT: [[TMP2:%.*]] = sub nsw i32 -2, [[TMP0:%.*]]
; CHECK-NEXT: [[TMP3:%.*]] = tail call range(i32 0, 33) i32 @llvm.ctlz.i32(i32 [[TMP2]], i1 false)
; CHECK-NEXT: [[TMP4:%.*]] = sub nsw i32 0, [[TMP3]]
; CHECK-NEXT: [[TMP5:%.*]] = and i32 [[TMP4]], 31
; CHECK-NEXT: [[TMP6:%.*]] = shl nuw i32 1, [[TMP5]]
; CHECK-NEXT: ret i32 [[TMP6]]
;
%2 = sub nsw i32 -2, %0
%3 = tail call i32 @llvm.ctlz.i32(i32 %2, i1 false)
%4 = sub i32 32, %3
%5 = shl i32 1, %4
%6 = icmp ult i32 %0, -2
%7 = select i1 %6, i32 %5, i32 1
ret i32 %7
}

declare i32 @llvm.ctlz.i32(i32, i1 immarg)
declare i64 @llvm.ctlz.i64(i64, i1 immarg)
declare <4 x i32> @llvm.ctlz.v4i32(<4 x i32>, i1)
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